Flash producing projectile



Inventors Benoit Jean;

James A. Park, Ottawa, Ontario, Canada; George P. T. Wilenius, Pottersville, New Jersey Appl. No. 857,074

Filed Sept. 11, 1969 Continuation-impart of Ser. No. 744,010, July 11, 1968, abandoned, which is a continuation-in-part of Ser. No. 623,961, Mar. 17, 1967, abandoned. Computing Devices of Canada Limited, Ottawa, Ontario, Canada Patented Sept. 15, 1970 Assignee Computing Devices of Canada Limited,

Ottawa, Ontario, Canada FLASH PRODUCING PROJECTILE 7 Claims, 11 Drawing Figs.

US. Cl 102/87, 102/92. 1 244/3.l

Int. Cl. ..F42h 13/36, F42b 1 H18 Field of Search 162/38,

[56] References Cited UNITED STATES PATENTS 906,771 12/1908 Clotz 244/3.23 1,103,740 7/1914 Cooper.. 102/38 1,292,388 l/l9l9 Bowers..... 244/3.23 2,386,054 10/1945 MeGee..... 244/3.23 2,402,718 6/1946 Albree 244/3.1

OTHER REFERENCES Limiting Conditions for Jet Formation in High Velocity Collisions, by Walsh, Shreffler and Willig, Journal of Applied Physics, vol. 24, #349 (1953).

Primary ExaminerRobert F. Stahl Att0rney-Weir, Marshall, MacRae and Lamb ABSTRACT: A projectile having a central longitudinally ex tending passageway and a recess at the nose end of the projectile joining the passageway. The recess has a surface inclined inwardly and rearwardly towards the central passageway so that, upon impact on a target, a spray of projectile material and/or target material is directed into and along the central passageway to exit at the rear of the passageway. The fragments in the spray interact with one another and with the atmosphere to generate light. Because the recess and central passageway concentrate the spray and the interaction, the light generated is enhanced.

Patented Sept. 15, 1970 Sheet g of 3 INVENTOR bENoa dam (JAMES A. Pm

BY 6m: $211. muimus PATENT A GENT Patented Sept. 15, 1970 I Sheet OUPSDQJJQ NO JSFMNOUI...

BEBE ZUFSZDP 1 N V E WTOR Eamon (JEAN OAME5A. m 5026!; RT-Vswmue r PATENT AGENT Patented Sept. 15, 1970 3,528,373

Sheet i of 5 INVENT OR PATENT AGENT FLASH PRODUCING PROJECTIILIE CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of U.S. application Ser. No. 744,010, filed July 11, 1968, which is in turn a continuation-in-part of US. application Ser. No. 623,961, filed Mar. 17, 1967, both now abandoned.

BACKGROUND OF THE INVENTION The invention relates to a projectile which produces a light with impact of the projectile on a target. In particular it relates to a projectile where the light generated by the impact itself is enhanced by the structure and configuration of the projectile, and no burning charge is required.

It is desirable for some purposes to have a projectile which produces a noticeable or detectable amount of light when the projectile strikes a target. For example, it may be desirable to have a projectile fired from a gun produce light on impact with a target in order to detect when the target has been hit. The ability to detect hits permits an immediate assessment of the effectiveness of the gun fire. For another example, it may be desirable to have a projectile produce light on impact in order to determine range. Range would be determined by measuring the time of flight, knowing the flight path and flight velocities. For yet another example, it may be desirable to have a projectile produce light on impact with a target in order to subject the light to a spectral analysis to determine the target material.

In the past projectiles were known which produce light on impact, but these projectiles have carried a charge of flash powder and an igniter which ignited the powder when the projectile struck a target. This is a relatively expensive manner in which to produce light when a projectile strikes a target and it may not be feasible or convenient to incorporate flash powder and an igniter on smaller projectiles. The use of flash powder and an igniter would not, of course, provide the desired flash for spectral analysis as mentioned above. The present invention seeks to produce a sufficient flash of light without requiring flash powder in the projectile.

It is known that the impact of a high velocity projectile on a target may be accompanied by a bright flash of light having a short duration. Considerable work has been done on impacting blunt, spherical and pointed projectiles on various targets to study the flash. For example there is a description of one manner of producing a flash by impacting a sphere on a target and analyzing the flash in Spectral Analysis of the Impact of Ultra Velocity Copper Spheres into Copper Targets by Clark, Kodesch and Grow, University of Utah, TR OSR--l3 (September 1959), and there is a description of a study of impact flash under various conditions in a paper entitled Investigation of Impact Flash at Low Ambient Pressure" by MacCormack, Proceedings of the Sixth Symposium on Hypervelocity Impact, I963, Vol. II, Pt. 2, pp. 613-625. It has been demonstrated that the flash of light is caused by interaction of a spray of high speed particles with the atmosphere.

In a paper Explosives with Lined Cavities by Birkhoff, Macdougall, Pugh and Taylor, Journal of Applied Physics 19, 563 (1948) the explosive collapse of a conical liner is described. The generation'of a jet of material with the collision of two flat plates is predicted with the velocity of the jet decreasing with increasing angle between the colliding plates. It appears that the collision or impact of a cone or sphere on a flat target is basically analogous to the collision of two flat plates or to the explosive collapse of a conical liner as described by Birkhoff et al. The differences are mainly those of the geometry involved. Reference is made to FIG. 1 which illustrates the similarity between a two plate collision, a cone impacting on a target and a sphere impacting on a target.

Sometime later, in 1953, Walsh, Shreffler and Willig published a paper entitled Limiting Conditions for Jet Formation in High Velocity Collisions, Journal of Applied Physics 24, 349 1953) which described flat plate collisions at various angles and under various conditions. It was found that a limiting condition exists in the collision of flat plates where a jet or spray is not formed. The jetless collision is possible when the angle of collision, that is the angle between the two flat plates, is smaller than a certain angle. It was postulated that as long as the shock wave produced by the collision of the two plates stays at the point of impact or collision the shocked material is effectively held behind the two colliding surfaces and no jetting can result. If, however, one of the shock waves, that is the shock wave in either plate, moves ahead of the point of collision, shocked material can escape in the form of a jet. The theory is supported by examples.

As explained in the Walsh et al. reference, for a jetless collision, where the angle between the surface of the projectile and target is less than a critical value, very high pressures exist in the region between the two shock surfaces. A jetless configuration is shown in FIG. 2. The coordinate system is considered to move with the collision region, such that the junction of the planes containing the original surfaces of the target and projectile appears stationary. In FIG. 2 p and p are densities of the projectile and target material, U is the velocity of the projectile material measured in the plane of the projectile surface, and U is the velocity of the target material measured in the plane of the target surface. If a system of coordinates is chosen such that the target is stationary, then U is also the velocity of the coordinate system which is moving with the collision point. The pressure P must be constant across the contact surface, i.e. in the slipstream, which separates the two dissimilar systems. A breakdown of the jetless configuration must occur when either (P) or 0 reaches the critical angle associated with that stream. The critical angle for a jet to forrrnwould then be given by where P is the smaller of P and P and P, is the pressure as sociated with maximum possible deflection of the projectile stream and P is the pressure associated with maximum possible deflection of the target stream. These are referred to as critical pressures. According to Walsh et al. the critical angle for a jet to start is obtained for the value of the pressure behind the shock which will satisfy the equation p is the density of the shocked projectile material and p is the density of the projectile material.

There will, of course, be a similar equation associated with P The density of the shocked material can be calculated from the Rankine-l-lugoniot equation poW=p(W- where W is the shock wave velocity U is the particle velocity behind the shock no is the density of the unshocked material If conditions are such that jetting can occur at the collision point the pressure in the shocked material will have a low value.

Thus, by solving equation (2) above and using the pressure as in equation (I) an angle of collision where jetting will start can be determined. The theory is quite general and applies to any material in which a shock wave can be generated. As Walsh et al. points out on page 351i, it is only necessary that the collision be sufficiently violent to provide a shock wave.

A paper entitled Metallic Equations of State for Hypervelocity Impact, by Tillotson, General Atomic Report GA- 3216 (I962) ARPA #25l-Gl, is also of interest. It gives tables of values for various conditions of impact-with a considerable number of materials. The shock wave conditions for a number of different materials are available in this paper.

In a report dated 1963 on An Investigation of the Phenomena of Impact Flash and Its Potential Use in Hit Detection and Target Discrimination Technique by Gehring and Warnica, proceedings of the Sixth Symposium on Hypervelocity Impact, Vol. II, Pt. 2, pp. 627-681 there is a discussion of impact flash caused by impact of projectiles onto targets under various conditions. The level of the light generated by an ordinary projectile impacting a target is of a relatively low level when compared to the light generated by a projectile according to the present invention.

To summarize, light generated by impacting projectiles of blunt, spherical and conical configuration on targets under various conditions has been the subject of considerable study in the past. The spray or jet caused by the impact has been noted as being directed radially outwards from the point of impact, the velocity of the spray or jet having been found, under certain conditions, to be several times the velocity of impact. The light caused by impact is generally insufficient for purposes of hit detection, range determination, spectral analysis, and the like. A general theory has been developed, based on a study of impacting plates in a symmetrical collision, by which an angle may be determined below which there is no spray or jet and at which the spray or jet is at a maximum velocity. The theory applies to the impacting of any materials that can support a shock wave and the only requirement is that the impact must be sufficiently violent to generate a shock wave in at least one of the materials. The theory is supported by several examples. The same theory is applicable generally to collisions between a cone shaped, spherical shaped, or other shaped projectile on a target.

SUMMARY The projectile according to the present invention has a structure which concentrates and enhances the light generated when the projectile strikes a target. The structure is not complex and is easily manufactured. Because the projectile requires no flash powder or other burning material and no igniter, it is cheaper to make and is readily made in smaller sizes.

Thus, the invention is for a projectile comprising a cylindrical wall defining a longitudinal passageway. One end edge of the cylindrical wall has a surface inclined inwardly towards the passageway forming a recess communicating with the passageway. To achieve maximum enhancement of light generated, the angle the surface of the recess makes with a plane through the nose of the projectile must be within a few degrees of a critical collision angle.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. la-c is a sketch, to which reference has already been made, useful in describing the invention,

FIG. 2 is a drawing, to which reference has already been made, showing the conditions existing at a collision,

FIG. 3 is a sectional view of a projectile according to an embodiment of the invention,

FIGS. 4 and 5 are partial sectional views used in describing the manner in which the projectile functions,

FIGS. 6 and 7 are graphs of critical collision angle plotted against impact velocity, and is useful in explaining limitations of the invention,

FIG. 8 is a graph of relative intensity of light plotted against impact angle, and

FIG. 9 is a schematic diagram illustrating the manner in which embodiments of the projectile were evaluated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 3 there is shown a projectile 10 according to an embodiment of the invention. Projectile 10 comprises a wall 11 of cylindrical configuration defining a central passageway 12. At the rear or tail end of the projectile there is a recessed portion 14 which receives a disc-like pushing plate 15. The pushing plate 15 serves to close passageway 12 during firing, that is it prevents the gases escaping through passageway 12 as the projectile is driven along a barrel during firing. The pushing plate 15 is fitted so that it will fall away once the projectile leaves the barrel. To improve the aerodynamics of the projectile, there is a slight taper in the forward or nose portion of wall 11. The nose end edge of wall 11 has a recess formed therein. The recess is formed by surface 16 of the nose end edge which is inclined inwardly and rearwardly starting from the outer periphery of the end edge and extending to meet passageway 12. The recess formed by surface 16 is shown in the form of an inverted truncated cone and this is the preferred form, however under some circumstances it might be desirable to use a surface 16 having a slightly curved concave or convex configuration.

The manner in which projectile 10 enhances the generated light will now be described. Projectile 10 is fired from a gun. As it leaves the barrel of the gun, the pressure within the barrel which pressed pushing plate 15 into the recessed portion 14 is no longer present. In addition, the movement of projectile 10 through the atmosphere causes a pressure within passageway 12 which presses pushing plate 15 out of recess 14, and pushing plate 15 falls away.

The events which take place when the projectile first strikes a target are best described with reference to FIG. 4 which shows a sectional view of the nose portion of a projectile at the instant it strikes a target 17 where the plane of target 17 is at right angles to the path of the projectile. The outer periphery of the nose end edge strikes the target and creates a spray or jet 18 of fragments. The spray is created all around the periphery and is directed inwards towards a point 20 on the axis of passageway 12. There is, in effect, animplosion involving fragments of projectile and/or target material. The relative amounts of projectile material and of target material will, of course, depend on the actual materials of the projectile and target as is known.

Because the spray or jet 18 is directed from all around the periphery towards point 20 there is a concentration of spray material in the region of point 20 and this concentration of material which is moving at a high speed expands along passageway 12 towards the tail end of the projectile. As the projectile continues to move against the target, a wider band of the projectile nose end impacts against the target to provide more material for the jet or spray which exits at the rear of passageway 12. In a normal projectile having a pointed nose end, the fragments are all directed away from the point of impact and there is relatively little chance of the fragments hitting one another. If the fragments should hit one another, because they are proceeding roughly in the same direction, the velocity of impact will be small. With a projectile according to this invention, the fragments are all directed approximately towards a common point 20 at individual speeds of several times the speed of the projectile at impact. Fragments coming from opposite directions will collide at almost twice their individual speeds. The high speed collisions create a high temperature at the point of collision which tends to vaporize the fragments. The resulting fragments and vaporized fragments expand along passageway 12.

It is believed that several factors may contribute to the enhancement of light. For example, the following factors may contribute: there are more fragments colliding with one another, there is a high temperature created within the projectile, and the fragments interact with themselves and with the atmosphere at a higher speed. The preceding factors are with reference to a conventional pointed projectile. Regardless of which factors contribute to the enhancement of light, it appears that the peak amount of light generated is increased by appropriate design according to the invention and also the duration of the light generation is extended.

Referring now to FIG. 5, the nose portion of a projectile is shown as the first point just strikes a target 17 inclined at an angle to the normal. At this moment the spray of fragments will be in two jets indicated roughly as 21 and 22. It will be apparent that this will result in little enhancement of light as only a small portion of jet 22 enters the conical recess formed by surface 16. However, as the projectile continues along its path, a greater portion of the periphery of the nose end of the projectile will impact on the target. The jet 22 will, in effect, be started from an arc which spreads around the periphery, and the spray of material will be directed radially inwards towards the axis of passageway 12. Thus, as the projectile continues there will be more of the spray directed along passageway 12 and there will be an enhancement of the light.

It should be pointed out that the angle (indicated as A in FIG. 3) between the plane defined by the nose end of the projectile (which in this case is the plane of the target) and the surface 16 of the conical recess in the projectile has limits which are relatively critical. That is, the angle A or impact angle must be within a few degrees either side of a critical col lision angle a.

It will be recalled that in the aforementioned Walsh et al. reference a theory was developed for determining in any symmetrical collision the minimum angle at which a jet or spray would occur. It is necessary to know the density of the materi- 'als, the shock wave velocity, and the impact velocity (from which other velocities may be determined). The Walsh et al. reference discusses a symmetrical collision, that is where two plates are each moving towards one another. The critical angle is given by equation (I i.e.,

According to Walsh et al., the critical angle is obtained for the value of the pressure behind the shock which will satisfy equation 2).

P01 i n) m (F i )ll '1'P01' %1 (#1+ The projectile of the present invention does not involve a symmetrical collision, however the same theory is applicable and the same equations are involved. The values of the velocities are expressed in new terms. The values may be determined from the information available in the aforementioned Tillotson reference. It will be realized that U is a function of a and equation (2) must be solved as a transcendental equation and the critical angle determined for given conditions of velocity and material. By way of example, FIG. 6 is a graph of theoretical collision angle a, determined for a non-symmetrical collision as in a projectile according to this invention, plotted against impact velocity for a target material of aluminum with projectile of aluminum, copper and tungsten. It should be noted that the impact velocities in the graph extend to quite high values that are not normally encountered. For example, the velocity of a bullet fired from a rifle might be of the order of 2700 ft./sec. or about 0.8 km./sec. At these velocities there is relatively little difference in the critical collision angle a with different metal projectile materials, and it willbe seen that the collision angle a is very approximately 9.

It has been found that the values for the angle a determined experimentally agree fairly closely to the values for the theoretical or calculated collision angle a. FIG. 7 is a graph of the critical collision angle a plotted against impact velocity for a target material of aluminum with a copper projectile. The solid line represents the theoretical relationship as determined by computer solution of the transcendental equation (2) and the dashed line represents the relationship determined experimentally. Spot checks using brass or copper projectile on an aluminum target indicate that agreement between theoretical and experimental values of the collision angle (1 holds for other materials, with experimental values normally being slightly higher. The differences appear to be within the limits of experimental error. It is a relatively easy task to determine experimentally the critical collision angle for a particular projectile and target area.

Differentprojectiles were fired at a target under various circumstances to show that an enhancement of light was obtained, to show that this enhancement of light was obtained even when the target was not at right angles to the projectile, to show a relationship between changing angle A and the intensity of light generated, to show the effect of increased duration of light generated, and to show that enhancement of light took place in at least the visible and the red and near infrared regions.

Referring now to FIG. 8, there is shown a graph of relative light intensity plotted against impact angle. Copper projectiles impacted against an aluminum target were used to obtain the two curves shown. One curve represents the relative intensity of light generated when projectiles having different impact angles were fired at 3,300 ft./sec. (about 1 km./sec.) and impacted on the target, and the other curve represents the same relationship when using a firing velocity of 18,000 ft./sec. (about 5.5 km./sec.). It will be seen that the critical collision angle a is about 12 in the first curve and about 28 in the second. It will also be seen that both the curves are about 5 wide at 50 percent light intensity and about 12 and 15 wide at 10 percent light intensity. The light enhancement provided at a relative intensity of 10 percent is still quite significant as will be discussed hereinafter.

It will be seen from FIG. 8 that an impact angle A in the range of from the critical angle a less 25 to the critical angle a plus 2.5 will provide considerable increase in intensity, and an impact angle A in the range of from the critical angle a less 7.5 to the critical angle a plus 7.5 will provide a useful increase in intensity.

Some examples are given below and the equipment used in obtaining the figures given in the examples is partly shown in the schematic drawing of FIG. 9. FIG. 9 will be described briefly to provide a background for the subsequent discussion and examples.

In FIG. 9, the line 24 represents the path of the projectile to target 17. The target region is enclosed in a casing 25 which has an opening 26 for a camera. The projectile path 24 extends by a triggering means 27 which may be used in the actuation of the light measuring devices. More than one triggering means may be employed if desired and they would be located at convenient places. Mirrors 30 and 31 are placed on opposite sides of the projectile path 24 and are arranged to direct light from the target 17 onto two light measuring devices 32 and 33. In the following examples, light measuring device 32 is a photodiode device and light measuring device 33 is a photomultiplier device. A red filter 34, such as a Kodak Wratten No. 25A is interposed in the light path to photodiode device 32, and a neutral density filter 35 is interposed in the light path to photomultiplier device 33. It will, of course, be apparent that the light enhancement could have been measured over both the red portion and the visible portion of the spectrum as one measurement. It has been broken down into two regions to show that while there is enhancement in both regions of the spectrum, it may be greater in one region than the other.

The apparatus of FIG. 9 was used to measure characteristics of the light generated on impact using a projectile according to the invention having an impact angle (i.e. an angle of the surface 16 with respect to a plane through the end of the projectile) of 20. The critical collision angle a as determined experimentally is approximately l2.5. It should be noted from FIG. 8 that the impact angle A is such that the relative intensity obtained is only about 10 percent of that which could be achieved if the impact angle had been made equal to the criti-. cal angle. The projectile length was three times the diameter and the central passageway was one third the diameter. There was a slight taper to the nose portion so that the diameter of the nose was five-sixths the major diameter. The projectile was fired by a .30 calibre rifle at a velocity of approximately 2700 ft./sec. For purposes of comparison, the same characteristics of light generated on impact were measured for a blunt solid projectile fired from the same rifle at the same velocity.

EXAMPLE I in this example the target is oriented at right angles to the projectile path. Several materials were used. The signal voltage measured was corrected for filter attenuation.

BRASS PROJECTILE, SOFT ALUMINUM TARGET intensity generated by the projectile of the invention. For example, from the above measurements the enhancement is of the order of about 40, 100 and 4 for the visible portion of the spectrum, and about 2, 65 and 64 for the red portion of the spectrum. The enhancement would be greater if the impact angle A used was equal to the critical angle a. It will be seen that the enhancement may be greater in the visible portion of the spectrunTthari in the red portion, or it'my be greater in the red portion than the visible portion depending on the materials involved. Nevertheless, in each case there is enhancement of light in both the visible portion and the red portion of the spectrum. It would, of course, be feasible to detect the enhancement over the entire range including visible and red.

lt is important to realize that the intensity of the flash for the blunt projectile and for the projectile of the invention having a 20 impact angle are not greatly different, but that the duration of the flash is increased significantly. The increased duration of the flash remains substantially the same over the range of interest, that is over a range of impact angle A extending from about 7.5 below the critical angle a to 7.5 above the Photomultiplier Photodiode Max. Sig. Product Max. Sig. Product Vel., slg. duration, volt. sig. duration, volt. Pro]. ft.lsec. volt. sec. Xtime volt. p560. Xtime Blunt 2, 730 200 4 3X10 0. 012 4 0. 48 Fig. 1 2, 700 180 200 3. 6X10 0. 11 100 l. 1

BRASS PROJECTILE, MILD STEEL TARGET Photomultipller Photodiode Max. Sig. Product Max. Sig. Product VeL, sig. duration, volt. sig. duration, volt. Pro]. it.lsoc. volt. psec. Xtime volt tsec. Xtime Blunt-.- 2, 690 a 4 24 o. 006 4 V 0. 02 Fig. 1 2, 660 100 3Xl0 0. 013 100 1.3

MILD STEEL PROJECTILE, MILD STEEL TARGET Photomultipiier Photodiode Max. Sig. Product Max. Sig. Product Vel., slg. duration, volt. sig. duration, volt. Pro it.lsec. volt. sec. Xtime volt. see. Xtime Blunt 2, 590 70 4 2. 8X10 0. 012 4 0. 05 Fig. 1 2, 630 120 10 1. 2x10 0.016 200 3. 2

critical angle a. The peak intensity of the flash occurs, of

course, when the impact angle A is equal to the critical angle a as was described in connection with FIG. 8.

EXAMPLE [I In this example the conditions are the same as in Example I reference by Clark et al. The application of this to the present invention will be apparent to those skilled in the art.

We claim:

1. A projectile for enhancing the light generated on impact,

comprising:

a cylindrical wall of metal defining a central passageway extending longitudinally from a nose end to a tail end; the endedge of said.wall at said nose: end having a surface inclined inwardly and rearwardly forming a recess com- BRASS PROJECTILE, SOFT ALUMINUM TARGET Photomultiplier Photodiode Max. Sig. Prod. Max Sig. Prod. Vel sig. duration, volt. sig. duration, volt. Proj. Target IL/sec volt. ps X time volt usec. X time 90 2, 730 200 4 8 X 10 0. 012 4 0. 048 90 2, 700 180 200 3.6 X 10 0. 011 100 1. 1 70 2, 610 40 50 2.0 X 10 50 2, 620 50 7.5 X 10 2 0. 01 100 1. 0

BRASS PROJECTILE, MILD STEEL TARGET Photomultiplier Photodiode Max. Sig. Prod. Max. Sig. Prod. Vel sig. duration, volt. sig duration, volt. Proj. Target ItJsec volt. us. X time volt. 11.5%. X time 90 2, 690 6 4 24 0. 006 4 0. 02 90 2, 660 100 3 X 10 3 0. 013 100 l. 3 70 2, 710 4 50 2 X 10 3 0. 004 100 0. 4 50 2,650 0 o MILD STEEL PROJECTILE, MILD STEEL TARGET Photomultiplier Photodlode Max. Sig. Prod. Max. Sig. Prod. VeL, slg. duration, volt. slg duration, volt. Proj. Target ItJsee. volt. time volt sec. X time 90 2, 590 70 4 2 8 X 10 3 0. 012 4 0. 05 90 2,630 120 10 l 2 X 10 0. 016 200 3. 2 70 2, 610 20 70 1 4 X 10 3 0. 012 70 0. S 50 2, 620 0 It will be seen that the projectile of this invention provides an enhancement of light even when the projectile does not strike a target at right angles. With the materials in the example there is an enhancement of the light at least to a 70 angle and probably considerably farther.

It is believed that a projectile has been described which is capable of enhancing the light generated on impact by a considerable amount as compared to a blunt projectile. The light generated is of short duration (i.e. less than a millisecond) and of high intensity and the flash of light made by a projectile according to the invention can be distinguished from normal background radiation on a clear day. The photomultiplier 33 as used to measure light intensity in Examples I and II will collect about 10 rnicrowatts of radiation from the sky on a dull day and about 1000 microwatts of radiation from a clear sky. The projectile of this invention may cause radiation of the order of 1000 microwatts which is comparable to that of the clear sky radiation. Because the flash generated by the projectile is of short duration it may be easily recognized from a steady background even on a clear day.

The size and shape of the projectile, the longitudinal passageway and the recess in the nose end may be varied from the sizes and shapes given in the examples and a light enhancement achieved as will be apparent to those skilled in the art.

It has been found that the light intensity of a flash generated by an impact using a projectile according to this invention is often sufficient for remote spectral analysis to determine target material. A discussion of spectral analysis of impact flash under certain conditions appears in the aforementioned municating with said passageway to direct fragments of material resulting from impact inwardly towards a point on the axis of said passageway and along said passageway to exit at thetail end; and

the angle defined by said. surface and a plane through said nose end at right angles to said passageway being in the range of angles defined by the critical collision angle plus and minus 7.5".

2. A projectile as defined in claim 1 in which said cylindrical wall of metal is selectedfrom a group of materials consisting of brass, copper and iron.

3. A projectile for enhancing the light generated on impact with a target of metal from the group consisting of aluminum and iron, said projectile comprising:

a cylindrical wall of metal selected from the group consisting of brass, copper and iron, said .wall defining a central passageway extending longitudinally from a nose end to a tail end;

the end edge of said wall at said nose end having a surface inclinedinwardly' andrearwardly forming a recess communicating with said passageway to direct fragments of material resulting from 'impact inwardly towards and along said passageway to exit at said tail end; and

the angledefined by .saidsurface anda plane through said nose endat right angles to said passageway being the critical collision angle for the'estimated impact velocity plus or minus 7.5.

4. A projectile as defined in claim 3 in which the intended target is aluminum and the projectile is of copper.

5. A projectile as defined in claim 4 in which the estimated impact velocity of the projectile is about 1 kmJsec. and the said angle defined by said surface and a plane through said nose is about 12.

6. A projectile as defined in claim 4 in which the estimated impact velocity of the projectile is about 5.5 km./sec. and the said angle defined by said surface and a plane through said nose end is about 28.

7. A projectile for enhancing the light generated on impact comprising:

a cylindrical wall of metal selected from the group consisting of brass, copper and iron, said wall defining a central passageway extending longitudinally from a nose end to a tail end;

the end edge of said wall at said nose end having a surface inclined inwardly and rearwardly forming a recess communicating with said passageway to direct fragments of 1-2 material resulting from impact inwardly towards and along said passageway to exit at the tail end; the angle defined by said surface and a plane through said nose end at right angles to said passageway being deter- 5 mined by optimizing the transcendental equation below d P P(PpmUoi'-') 0 )[I 1-poi-Uo1 -P(I 1+ where P is the pressure at the contact surface #1?(P1/Po|)-'1 2; 1s the density of the shocked pro ectile matenal p is the density of the projectile material U is the velocity of the projectile material measured in the plane of the projectile surface,

and is a. function of said angle. 

